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Using Flex-Algo for Segment Routing (SR) based Virtual Transport Network (VTN)
draft-zhu-lsr-isis-sr-vtn-flexalgo-05

Document Type Active Internet-Draft (individual)
Authors Yongqing Zhu , Jie Dong , Zhibo Hu
Last updated 2022-07-11
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draft-zhu-lsr-isis-sr-vtn-flexalgo-05
LSR Working Group                                                 Y. Zhu
Internet-Draft                                             China Telecom
Intended status: Standards Track                                 J. Dong
Expires: 12 January 2023                                           Z. Hu
                                                     Huawei Technologies
                                                            11 July 2022

Using Flex-Algo for Segment Routing (SR) based Virtual Transport Network
                                 (VTN)
                 draft-zhu-lsr-isis-sr-vtn-flexalgo-05

Abstract

   Enhanced VPN (VPN+) aims to provide enhanced VPN service to support
   some application's needs of enhanced isolation and stringent
   performance requirements.  VPN+ requires integration between the
   overlay VPN connectivity and the characteristics provided by the
   underlay network.  A Virtual Transport Network (VTN) is a virtual
   underlay network which has a customized network topology and a set of
   network resources allocated from the physical network.  A VTN could
   be used as the underlay for one or a group of VPN+ services.

   The topological constraints of a VTN can be defined using Flex-Algo,
   a mechanism to provide distributed constraint-path computation.  In
   some network scenarios, each VTN can be associated with a unique
   Flex-Algo, and the set of network resources allocated to a VTN can be
   instantiated as layer-2 sub-interfaces or member links of the layer-3
   interfaces.  This document describes the mechanisms to build the
   Segment Routing (SR) based VTNs using SR Flex-Algo and IGP L2 bundles
   with minor extensions.  This document updates RFC 8668 by defining a
   new flag in the Parent L3 Neighbor Descriptor in the L2 Bundle Member
   Attributes TLV.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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   This Internet-Draft will expire on 12 January 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Advertisement of SR VTN Topology Attributes . . . . . . . . .   4
   3.  Advertisement of SR VTN Resource Attributes . . . . . . . . .   4
   4.  Forwarding Plane Operations . . . . . . . . . . . . . . . . .   6
   5.  Scalability Considerations  . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   Enhanced VPN (VPN+) is an enhancement to VPN services to support the
   needs of new applications, particularly including the applications
   that are associated with 5G services.  These applications require
   enhanced isolation and have more stringent performance requirements
   than that could be provided with existing overlay VPN techniques.
   Thus these properties require integration between the underlay and
   the overlay networks.  [I-D.ietf-teas-enhanced-vpn] specifies the
   framework of enhanced VPN and describes the candidate component
   technologies in different network planes and layers.  An enhanced VPN
   may be used for 5G transport network slicing, and will also be of use
   in other generic scenarios.

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   To meet the requirement of enhanced VPN services, a number of virtual
   transport networks (VTN) can be created, each with a subset of the
   underlay network topology and a set of network resources allocated
   from the underlay network to meet the requirement of a specific VPN+
   service or a group of VPN+ services.  Another possible approach is to
   create a set of point-to-point paths, each with a set of network
   resource reserved along the path, such paths are called Virtual
   Transport Paths (VTPs).  Although using a set of dedicated VTPs can
   provide similar characteristics as VTN, it has some scalability
   issues due to the per-path state in the network.

   [I-D.ietf-spring-resource-aware-segments] introduces resource
   awareness to Segment Routing (SR) [RFC8402].  As described in
   [I-D.ietf-spring-sr-for-enhanced-vpn], the resource-aware segment
   identifiers (SIDs) can be used to build VTNs with the required
   network topology and network resource attributes to support VPN+
   services.  With a segment routing based data plane, SIDs can be used
   to represent both the topology and the set of network resources
   allocated by network nodes to a VTN.  The SIDs of each VTN together
   with its associated topology and resource attributes need to be
   distributed using the control plane.

   [I-D.dong-lsr-sr-enhanced-vpn] defines the IGP mechanisms and
   extensions to provide scalable SR based VTNs.  The mechanism in
   [I-D.dong-lsr-sr-enhanced-vpn] allows flexible combination of the
   topology and resource attribute to provide a relatively large number
   of VTNs.  In some network scenarios, the number of required VTNs may
   be small, thus a solution to provide a small number of VTNs may also
   be desired.

   This document describes a mechanism to build the SR based VTNs using
   SR Flex-Algo [I-D.ietf-lsr-flex-algo] and IGP L2 bundle [RFC8668]
   with minor extensions.  With this mechanism, each VTN is associated
   with a unique Flex-Algo, and the set of network resources allocated
   to the VTN is instantiated using layer-2 sub-interfaces or layer-2
   member links of the L3 interfaces.  It can provide a relatively small
   number of VTNs, and can be considered as a transitional solution for
   the VTN deployment.

   This document updates [RFC8668] by defining a new flag in the Parent
   L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV.
   [RFC8668] states that all bit fields not defined in that document
   "MUST be set to zero on transmission and ignored on receipt".
   Section 3 of this document defines a new flag and specifies both when
   it is set and how it should be processed.

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1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Advertisement of SR VTN Topology Attributes

   [I-D.ietf-lsr-flex-algo] specifies the mechanism to provide
   distributed constraint-path computation, and the usage of SR-MPLS
   prefix-SIDs and SRv6 locators for steering traffic along the
   constrained paths.

   The Flex-Algo Definition (FAD) is the combination of calculation-
   type, metric-type and the topological constraints used for path
   computation.  According to the network nodes' participation of a
   Flex-Algo, and the rules of including or excluding Admin Groups (i.e.
   colors) and Shared Risk Link Groups (SRLGs), the topology of a VTN
   can be described using the associated Flex-Algo.  If each VTN is
   associated with a unique Flex-Algo, the Flex-Algo identifier could be
   reused as the identifier of the VTN in the control plane.

   With the mechanisms defined in[RFC8667] [I-D.ietf-lsr-flex-algo], SR-
   MPLS prefix-SID advertisement can be associated with a specific
   topology and a specific algorithm, which can be a Flex-Algo.  This
   allows the nodes to use the prefix-SIDs to steer traffic along
   distributed computed constraint paths according to the associated
   Flex-Algo in a particular topology.

   [I-D.ietf-lsr-isis-srv6-extensions] specifies the IS-IS extensions to
   support SRv6 data plane, in which the SRv6 locators advertisement is
   associated with a topology and a specific algorithm, which can be a
   Flex-Algo.  This allows the nodes to use the SRv6 locators to steer
   traffic along distributed computed constraint paths according to the
   associated Flex-Algo in a particular topology.  In addition,
   topology/algorithm specific SRv6 End SIDs and End.X SIDs can be used
   to enforce traffic over the Loop-Free Alternatives (LFA) computed
   backup paths.

3.  Advertisement of SR VTN Resource Attributes

   Each VTN can be allocated a set of dedicated network resources on
   different network nodes and links.

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   In order for a network controller or the ingress nodes to perform
   constraint based path computation for each VTN, the resource
   attributes of each VTN need to be advertised.  This way, the network
   controller or the ingress node can compute an SR TE path in a VTN by
   taking both the Flex-Algo constraints and the resource attributes of
   the VTN into consideration.

   IS-IS L2 Bundle [RFC8668] was defined to advertise the link
   attributes of the layer-2 bundle member links.  In this section, it
   is extended to advertise the set of network resource attributes
   associated with different VTNs on a layer-3 link.

   The layer-3 link must have the capability of partitioning the link
   resources into different subsets for the different VTNs it
   participates in.  It may or may not be a bundle of layer-2 links to
   achieve this.  One partition of the link resources can be
   instantiated as a layer-2 sub-interface, which can be seen as a
   virtual layer-2 member link of the layer-3 link.  If the layer-3 link
   is a layer-2 link bundle, it is possible that the set of link
   resources allocated to a specific VTN is provided by one or multiple
   physical layer-2 member links.

   A new flag "E" (Exclusive) is defined in the flag field of the Parent
   L3 Neighbor Descriptor in the L2 Bundle Member Attributes TLV (25).

                0 1 2 3 4 5 6 7
               +-+-+-+-+-+-+-+-+
               |P|E|           |
               +-+-+-+-+-+-+-+-+

   E flag: When the E flag is set, it indicates each member link under
   the Parent L3 link is used exclusively for one VTN, and load sharing
   among the member links is not allowed.  When the E flag is clear, it
   indicates load balancing and sharing among the member links are
   allowed.

   Note that legacy implementations of [RFC8668] will set the E flag to
   zero (clear) meaning that load balancing among component links is the
   default behavior.  Further, when a legacy implementation receives an
   E flag that is set, it will ignore the flag and so will assume that
   load balancing among component links is allowed even when the sender
   has requested it to not be used.  The Flex-Algo associated with the
   VTN can be defined that only nodes which support the E flag are
   included in the constraint-based path computation and packet
   forwarding of the VTN.

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   For each virtual or physical layer-2 member link, the Admin Groups
   (AG) or Extended Admin Group (EAG) attribute MUST be advertised using
   the mechanisms as defined in [RFC8668].  This is for the correlation
   between the Flex-Algo specific forwarding entries and the layer-2
   member link.  Other TE attributes as defined in [RFC5305] such as the
   Maximum Link Bandwidth attribute MAY also be advertised for the
   constraint-based path computation performed by the controller or the
   ingress nodes.  The SR-MPLS Adj-SIDs or SRv6 End.X SIDs associated
   with each of the virtual or physical layer-2 member links MUST be
   advertised according to [RFC8668] and [I-D.dong-lsr-l2bundle-srv6].

   In order to correlate the virtual or physical layer-2 member links
   with the Flex-Algo ID which is used to identify the VTN, each VTN is
   assigned with a unique Admin Group (AG) or Extended Admin Group
   (EAG), and the virtual or physical layer-2 member links associated
   with this VTN are configured with the AG or EAG assigned to the VTN.
   The AG or EAG of the parent layer-3 link is set to the union of all
   the AGs or EAGs of its virtual or physical layer-2 member links.  In
   the definition of the Flex-Algo corresponding to the VTN, it MUST use
   the Include-Any Admin Group rule with only the AG or EAG assigned to
   the VTN as the link constraints, the Include-All Admin Goup rule or
   the Exclude Admin Group rule MUST NOT be used.  This is to ensure
   that the layer-3 link is included in the Flex-Algo constraint based
   path computation for each VTN it participates in.

4.  Forwarding Plane Operations

   For the SR-MPLS data plane, a prefix SID is associated with the paths
   calculated using the Flex-Algo corresponding to a VTN.  An outgoing
   layer-3 interface is determined for each path.  In addition, the
   prefix-SID also steers the traffic to use the virtual or physical
   layer-2 member link which is associated with the VTN on the outgoing
   layer-3 interface for packet forwarding.  A forwarding entry MUST be
   installed in the forwarding plane using the MPLS label that
   corresponds to the Prefix-SID associated with the Flex-algorithm
   corresponding to the VTN.  The Adj-SIDs associated with the virtual
   or physical member links of a VTN can be used together with the
   prefix-SIDs of the same VTN to build SR-MPLS TE paths under the
   topological and resource constraints of the VTN.

   For the SRv6 data plane, an SRv6 Locator is a prefix which is
   associated with the paths calculated using the Flex-Algo
   corresponding to a VTN.  An outgoing Layer-3 interface is determined
   for each path.  In addition, the SRv6 Locator prefix also steers the
   traffic to use the virtual or physical layer-2 member link which is
   associated with the VTN on the outgoing layer-3 interface for packet
   forwarding.  A forwarding entry for the SRv6 Locator prefix MUST be
   installed in the forwarding plane for the Flex-algorithm

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   corresponding to the VTN.The End.XU SIDs associated with the virtual
   or physical member links of a VTN can be used together with other
   types of SRv6 SIDs of the same VTN to build SRv6 paths under the
   topological and resource constraints of the VTN.

5.  Scalability Considerations

   The mechanism described in this document assumes that each VTN is
   associated with a unique Flex-Algo, so that the Flex-Algo IDs can be
   reused to identify the VTNs in the control plane.  While this brings
   the benefit of simplicity, it also has some limitations.  For
   example, it means that even if multiple VTNs share the same
   topological constraints, they still need to be identified using
   different Flex-Algo IDs in the control plane, then independent path
   computation needs to be executed for each VTN.  The number of VTNs
   supported in a network may be dependent on the number of Flex-Algos
   supported, which is related to the number of Flex-Algos supported in
   the protocol (which is 128) and the control plane overhead on network
   nodes.  The mechanism described in this document is applicable to
   network scenarios where the number of required VTN is relatively
   small.  A detailed analysis about the VTN scalability and the
   possible optimizations for supporting a large number of VTNs can be
   found in [I-D.dong-teas-nrp-scalability].

6.  Security Considerations

   This document introduces no additional security vulnerabilities to
   IS-IS.

   The mechanism proposed in this document is subject to the same
   vulnerabilities as any other protocol that relies on IGPs.

7.  IANA Considerations

   This document does not request any IANA actions.

8.  Acknowledgments

   The authors would like to thank Zhenbin Li, Peter Psenak, Adrian
   Farrel and Gyan Mishra for the review and discussion of this
   document.

9.  References

9.1.  Normative References

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   [I-D.dong-lsr-l2bundle-srv6]
              Dong, J. and Z. Hu, "Advertising SRv6 SIDs for Layer 2
              Bundle Member Links in IGP", Work in Progress, Internet-
              Draft, draft-dong-lsr-l2bundle-srv6-01, 24 October 2021,
              <https://www.ietf.org/archive/id/draft-dong-lsr-l2bundle-
              srv6-01.txt>.

   [I-D.ietf-lsr-flex-algo]
              Psenak, P., Hegde, S., Filsfils, C., Talaulikar, K., and
              A. Gulko, "IGP Flexible Algorithm", Work in Progress,
              Internet-Draft, draft-ietf-lsr-flex-algo-20, 18 May 2022,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-flex-algo-
              20.txt>.

   [I-D.ietf-lsr-isis-srv6-extensions]
              Psenak, P., Filsfils, C., Bashandy, A., Decraene, B., and
              Z. Hu, "IS-IS Extensions to Support Segment Routing over
              IPv6 Dataplane", Work in Progress, Internet-Draft, draft-
              ietf-lsr-isis-srv6-extensions-18, 20 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-lsr-isis-srv6-
              extensions-18.txt>.

   [I-D.ietf-spring-resource-aware-segments]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Introducing Resource Awareness to SR
              Segments", Work in Progress, Internet-Draft, draft-ietf-
              spring-resource-aware-segments-04, 5 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-spring-
              resource-aware-segments-04.txt>.

   [I-D.ietf-spring-sr-for-enhanced-vpn]
              Dong, J., Bryant, S., Miyasaka, T., Zhu, Y., Qin, F., Li,
              Z., and F. Clad, "Segment Routing based Virtual Transport
              Network (VTN) for Enhanced VPN", Work in Progress,
              Internet-Draft, draft-ietf-spring-sr-for-enhanced-vpn-02,
              5 March 2022, <https://www.ietf.org/archive/id/draft-ietf-
              spring-sr-for-enhanced-vpn-02.txt>.

   [I-D.ietf-teas-enhanced-vpn]
              Dong, J., Bryant, S., Li, Z., Miyasaka, T., and Y. Lee, "A
              Framework for Enhanced Virtual Private Network (VPN+)
              Services", Work in Progress, Internet-Draft, draft-ietf-
              teas-enhanced-vpn-10, 6 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-teas-enhanced-
              vpn-10.txt>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <https://www.rfc-editor.org/info/rfc5305>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

   [RFC8668]  Ginsberg, L., Ed., Bashandy, A., Filsfils, C., Nanduri,
              M., and E. Aries, "Advertising Layer 2 Bundle Member Link
              Attributes in IS-IS", RFC 8668, DOI 10.17487/RFC8668,
              December 2019, <https://www.rfc-editor.org/info/rfc8668>.

9.2.  Informative References

   [I-D.dong-lsr-sr-enhanced-vpn]
              Dong, J., Hu, Z., Li, Z., Tang, X., Pang, R., JooHeon, L.,
              and S. Bryant, "IGP Extensions for Scalable Segment
              Routing based Enhanced VPN", Work in Progress, Internet-
              Draft, draft-dong-lsr-sr-enhanced-vpn-07, 29 January 2022,
              <https://www.ietf.org/archive/id/draft-dong-lsr-sr-
              enhanced-vpn-07.txt>.

   [I-D.dong-teas-nrp-scalability]
              Dong, J., Li, Z., Gong, L., Yang, G., Guichard, J. N.,
              Mishra, G., Qin, F., Saad, T., and V. P. Beeram,
              "Scalability Considerations for Network Resource
              Partition", Work in Progress, Internet-Draft, draft-dong-
              teas-nrp-scalability-02, 16 May 2022,
              <https://www.ietf.org/archive/id/draft-dong-teas-nrp-
              scalability-02.txt>.

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Authors' Addresses

   Yongqing Zhu
   China Telecom
   Email: zhuyq8@chinatelecom.cn

   Jie Dong
   Huawei Technologies
   Email: jie.dong@huawei.com

   Zhibo Hu
   Huawei Technologies
   Email: huzhibo@huawei.com

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